Glass sheets utilizing the photocatalytic function exerted by titanium oxide films are manufactured and commercially sold as so-called “self-cleaning glass”. As is well known, the photocatalytic function of titanium oxide films causes decomposition of organic substances adhered to the surfaces of glass sheets and thereby weakening of the adhesiveness of the organic substances, thus allowing the organic substances to be washed away by rainwater or the like.
The refractive index of titanium oxide (about 2.5 in the case of the anatase type) is higher than the refractive index of glass. For this reason, the formation of a titanium oxide film on a surface of a glass sheet increases the light reflectance of the glass sheet. Therefore, in view of uses for which increase in light reflectance is not desired (e.g., uses in solar cells or greenhouses), a photocatalyst film capable of suppressing the increase in light reflectance has been proposed. In this photocatalyst film, the refractive index of the film is decreased by having the film contain not only titanium oxide particles but also particles of silicon oxide having a lower refractive index than titanium oxide. A sufficient decrease in the refractive index of the photocatalyst film allows a decrease in reflectance of light incident on the glass sheet.
WO 2011/021383 A1 (Patent Literature 1) discloses a glass article including a glass sheet and a photocatalyst film formed on a surface of the glass sheet, the glass article being an improved glass article made suitable for long-term use by enhancing the film strength of the photocatalyst film while maintaining the photocatalytic function and the reflection-reducing function of the photocatalyst film.
As shown in FIG. 11, in a photocatalyst film 11 disclosed in Patent Literature 1, some silicon oxide particles denoted by 17 are situated in the uppermost portion of the film and are connected to a glass sheet 12 via silicon oxide particles 16 situated in the lower portion. The silicon oxide particles 16 and 17 and the glass sheet 12 are firmly fixed together by a binder material. Titanium oxide particles 15 are situated lower than the top portions of the silicon oxide particles 17. Such a film structure is suitable for increasing the film strength and for maintaining the function of the photocatalyst film by preventing detachment of titanium oxide.